教程内容：
CentOS 8.x默认使用的是firewall作为防火墙，这里改为iptables防火墙。
1、关闭firewall：
systemctl stop firewalld.service #停止firewall
systemctl disable firewalld.service #禁止firewall开机启动
systemctl mask firewalld
systemctl stop firewalld
yum remove firewalld2、安装iptables防火墙
yum install iptables-services #安装
vi /etc/sysconfig/iptables #编辑防火墙配置文件
# sample configuration for iptables service
# you can edit this manually or use system-config-firewall
# please do not ask us to add additional ports/services to this default configuration
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 443 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 3306 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
COMMIT
:wq! #保存退出
systemctl restart iptables.service #最后重启防火墙使配置生效
systemctl enable iptables.service #设置防火墙开机启动
/usr/libexec/iptables/iptables.init restart #重启防火墙评论（0）条
